1. Field of the Invention
The present invention relates to a polymer latex which can be prepared into powder having excellent impact resistance and flow properties, as well as a preparation method thereof. Furthermore, the present invention relates to polymer powder obtained by spray-drying the polymer latex, and a resin composition containing the polymer powder as an impact modifier.
2. Description of the Prior Art
Polymer resins or their mixtures are used to manufacture a variety of molded articles. However, such resins mostly show high brittleness only by themselves, and hence have no impact resistance required in final products. To overcome such a shortcoming, additives are generally used in the processing of thermoplastic resins. The most typical examples of the thermoplastic resins include polyvinyl chloride (PVC) resin. Generally, impact-resistance improvers or impact modifiers, such as methyl methacrylate-butadiene-styrene (MBS) resins, chlorinated polyethylene (CPE) resins or acrylic resins, are added to improve the impact resistance of PVC. Of such additives, the acrylic resins are particularly excellent in weather resistance, and so widely used as impact modifiers for exterior PVC products which are exposed to sunlight for relatively a long time.
The impact modifiers are generally supplied in the form of powders which mostly have a core-shell structure. Generally, the cores are made of a rubbery polymer with low glass transition temperature, and the shells are made of a polymer with a relatively high glass transition temperature. In this case, in order to increase the impact strength of the resins, it is preferable to increase the content of a rubbery polymer in the impact modifiers, thus increasing the impact resistance of the impact modifiers. However, if the content of the rubbery polymer in the impact modifiers is increased, the amount of the shell material in the structure of the impact modifiers will be decreased so that portions of the cores which are not surrounded by the shells will be large, resulting in an increase in the stickiness of the impact modifier particles. Accordingly, it will be difficult for conventional isolation methods to prepare powders with excellent flow properties from an emulsion having the impact modifier polymer particles dispersed therein.
Methods of obtaining impact modifiers in a powder form by isolating particles from an emulsion having the impact modifier polymer particles dispersed therein can be broadly into two categories. The first is to obtain powders by coagulation and drying, and the second is to obtain powders by spray drying.
In the case of the coagulation and drying, electrolytes, organic acids or inorganic acids are added to a prepared emulsion containing the impact modifiers so as to prepare coagulated slurry. The coagulated slurry is washed and filtered, followed by drying in a dryer, thus obtaining impact modifier powders.
In the case of the spray drying, an impact modifier-containing emulsion is sprayed in a drier containing hot air so as to evaporate moisture, thus obtaining impact modifier powders. Although the spray drying is a very economical and simple method to obtain the impact modifier powders, it is inconvenient in that the impact modifier particles can easily adhere to the wall surface of the spray dryer, and a sprayer should be frequently washed such that the sprayer outlet is not clogged by the sticking of the impact modifiers. This problem becomes severe, particularly when an emulsion containing impact modifiers with high rubber content is spray-dried.
In an attempt to solve this problem, U.S. Pat. No. 4,278,576 discloses a method of using stearate coated calcium carbonate as a flow improver in spray-drying the impact modifiers with high rubber content. Also, this patent discloses co-spray drying the impact modifier having a rubber content of 88%, with 7% by weight of the flow improver.
However, there is still no disclosure of an example of either an acrylic polymer latex having low viscosity together with a rubber content of more than 90 wt %, or a diene polymer latex with low viscosity together with a rubber content of more than 40 wt %, which can be prepared into powders with excellent long-term workability and flow properties.